This invention relates to an interferometer and more particularly to a hand held phase shifting diffraction moire interferometer.
The technique of diffraction interferometry and diffraction grating interferometry, upon which it is based, is directly sensitive to in-plane displacements, provides excellent, variable resolution, and very high quality data. Diffraction moire interferometry uses a reflective-type diffraction grating (i.e., a specimen grating) which is fixed to an object under study and illuminated by two mutually coherent collimated beams of light at precise incident angles. The specimen grating interacts with each of these beams to generate (diffract) secondary beams which exit the specimen grating. Interference fringes, sometimes called moire patterns, representing a contour map of in-plane displacements can be observed in the combined secondary beams.
Comparison of fringes before and after loading can be used to determine loading induced displacements. Additionally, the comparison of fringe patterns taken over a period of time can be used for tagging purposes, that is, verifying the identification of the object. (see U.S. Pat. No. 5,003,600, Deason et al.) Analysis of very complex, arbitrary fringe patterns by the conventional fringe tracing techniques is time consuming and difficult to automate. Phase shifting methods, whereby the phase of one incident beam is shifted relative to the other incident beam, greatly reduces the difficulty of analyzing interferograms and provides values at more points on the two dimensional interferogram.
A particular technique for diffraction moire interferometry is demonstrated by D. Post and W. A. Baracat, "High-sensitivity Moire Interferometry--A Simplified Approach," Experimental Mechanics, March 1981, pp. 100-104. This technique utilizes a single collimated beam, part of which strikes the specimen at a specific incident angle while the other part of the beam strikes a mirror located perpendicular to the specimen and is then reflected upon the specimen at the same, but opposite incident angle.
Normally, the set-up for diffraction moire interferometer measurements has been under very controlled laboratory conditions requiring several hours for a very experienced operator to perform. The standard method requires complex and cumbersome optical set-ups, including lasers, spatial filters, collimators, beamsplitters, mirrors and path-matching arrangements. Because of this involved process, diffraction moire interferometers have been limited in the locations where they could be operated and have required highly trained and experienced operators.
A portable diffraction moire interferometer exists (see U.S. Pat. No. 4,850,693, Deason et al.) which is compact, portable, convenient to use, and requires little user familiarity with the underlying concepts of diffraction moire interferometry. This device is approximately the size of a briefcase and weighs about 40 pounds. However, further reductions in size and weight, simplification of data acquisition requirements, and the hand held convenience of the present invention, will enable the additional utilization of diffraction interferometry at locations, and under conditions, previously unsuitable for the portable diffraction moire interferometer.
It is an object of this invention to provide an apparatus for the measurement of deformed gratings and determination of material distortion or strain in non-laboratory situations.
It is another object of this invention to provide a diffraction moire interferometer which is hand held, convenient to use, and requires little user familiarity with the underlying concepts of diffraction interferometry.
It is another object of this invention to provide a method for simplification of data acquisition requirements for diffraction moire interferometry.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.